The present invention relates to automated discrete fluid sample analyzers and, more particularly, to sample preparation in automated discrete fluid sample analyzers.
Analysis of a fluid sample typically requires measuring a volume of the sample, applying one or more reagents, buffers, or catalysts to the sample, and assaying the sample using a device such as a calorimeter, photometer, or the like. For example, in the testing of fluids (e.g., water) for nitrate content, a predetermined amount of the fluid is combined with a predetermined amount of a buffer (e.g., ammonium chloride). The fluid sample is then applied to a catalyst, such as cadmium, to reduce the nitrates into nitrites. After reduction, the fluid sample is then assayed using a colorimeter to determine the nitrite content of the sample, which correlates to the nitrate content. Where a large number of samples are to be analyzed, manually performing this process can be time consuming, costly, and inconsistent.
Out of the need to reduce costs and improve consistency, Continuous Flow Analysis (CFA) was developed. With CFA, a series of fluid samples, each of which may be mixed with one or more reagents and/or buffers, are pumped through a tubing path. Each fluid sample within the tubing path is separated from adjacent fluid samples by air bubbles, which act to clean the tubing path between fluid samples. The tubing path may include a tube formed from a catalyst, such as cadmium, through which each fluid sample flows to reduce the fluid sample. The series of fluid samples continues along the tubing path where each fluid sample in the series is individually assayed using a device such as a calorimeter photometer, or the like.
The advent of CFA has improved productivity and consistency. Unfortunately, lengthy start-up and shut-down times make CFA time consuming for small analysis runs. In addition, operator technique in setting up and performing each analysis run plays a significant role in the quality of the data obtained using CFA.
To further reduce costs and improve consistency, automated discrete analysis techniques were developed. Automated discrete analysis techniques use computer-controlled automation to perform steps similar to those of the manual method. For example, a computer-controlled robotic arm may be used to position a probe of a pipettor to aspirate or deposit a fluid sample, buffer, or reagent into or out of any of a plurality of sample receptacles, a plurality of reagent receptacles, and a plurality of reaction cuvettes. This is a “discrete” analysis technique because each sample is deposited in a discrete reaction cuvette, which is then subjected to an assaying device such as a calorimeter photometer, or the like.
Automated discrete analysis techniques provide many advantages over CFA. First, because the analysis is discrete, certain variables associated with CFA (e.g., developing and maintaining correct bubble patterns, obtaining baseline readings, and carryover between samples) are eliminated. As a result, operator technique is not as critical with automated discrete analysis as it is with CFA. Moreover, because these variables are eliminated, consistency between analyses is improved. Second, the start-up and shut-down times for analysis runs is eliminated. Finally, because pipettors may be used in lieu of peristaltic pumps, variability due to pump tube elongation is eliminated.
However, for certain assays, discrete analysis requires that the samples be prepared either prior to placing them in the sample receptacle or that the run be stopped and the samples removed and prepared prior to continuing the run. For example, in the testing of fluids (e.g., water) for nitrate content, the samples must be reduced before they are assayed. With discrete analysis systems, this requires manually removing the sample, performing the reduction, and replacing the sample. This manual step can be time consuming and inconsistent.
One recent technique used to overcome this limitation is to place a cadmium tube in the fluid sample flow path within the pipettor. When the fluid sample, which is typically mixed with a buffer, is aspirated into the pipettor, the fluid sample comes in contact with the cadmium tube and the nitrates in the fluid sample are reduced.